1. Field of the Invention
The invention relates to a class D amplifier that converts PCM (Pulse Code Modulation) sound data to a PWM (Pulse Width Modulation) signal to be amplified and output, and particularly to a class D amplifier that is designed to reduce output noise.
2. Description of Related Art
As is well known, in this type of D class amplifier, jitter within the clocks for a PWM conversion appears as output noise as it is. An arithmetic error such as discarding lower bits in the digital process at a PWM conversion contributes to output noise. In a class D amplifier in prior art, as described in Japanese Patent Publication Sho 59-183510, an output of the class D amplifier is filtered through a low pass filter so that it may be converted to an analog signal to be supplied to a load (a speaker). At the same time, the analog signal is converted to a digital signal to be fedback to an input side. However, this type of processing has a drawback that it requires a high precision A/D (analog to digital) converter, which increases the number of components and makes a circuit complex and expensive.
An amplifier has been in actual use that converts PCM sound data (“sound” here means general sound such as musical sound, without being limited to the so-called sound) to an analog signal, which in turn is converted to a PWM signal. This amplifier, which performs analog processing, can easily feedback an output. But, there is a problem in that when PWM is performed by an analog processing, it is susceptible to an extraneous signal and a signal such as input digital data.
An A/D converter is required in order to perform feedback by digital processing, which raises a problem that the cost has gone up and at the same time a complex feedback design has become difficult. Moreover, measuring power output by a count feedbacking it only deteriorates the quality of a feedback signal because of quantization noise caused at the time when a pulse width is counted and noise induced by power. Accordingly, there is another problem that output quality has not been improved.
With regard to prior art literature for a class D amplifier, Japanese Patent Publication No. 2003-249825 is known.
In the situation, there is a need to solve the problems.